The scrapie prion protein (PrPSc) is the principal component of the infectious agent (prion) responsible for a group of fatal neurodegenerative conditions, including Creutzfeldt-Jakob disease, kuru and Gerstmann-Straussler-Scheinker disease in man, and scrapie and bovine spongiform encephalopathy in animals. These disorders arise in one of three ways: by infection, by inheritance, and sporadically. During prion infection, PrPSc is formed by posttranslational modification of a glycolipid-anchored membrane protein of the host called PrPC. Genetic cases result from one of several mutations in the host PrP gene. Mutant PrP can cause neurodegeneration in humans and transgenic mice with little or no detectable production of PrPSc. While PrPSc has been the subject of intensive study, much less attention has been paid to mutant PrP, whose pathophysiological mechanism is unknown. The purpose of the present application is to explore several interrelated questions concerning the cellular properties of mutant PrP, with a view to understanding how mutant PrP causes neural degeneration, as well as astrocytic glial proliferation. Our investigations will be extended to involve the Alzheimer amyloid precursor protein (APP) as well. This proposal builds upon previous studies of mutant PrP and APP expression in transgenic mice, and our own detailed analyses of the fates of neuronal and non-neuronal cells in cultured, differentiated murine embryonic carcinoma cells expressing mutant PrP. An experimental paradigm developed in our laboratory has enabled us to study the dramatically different effects of mutant versus wild-type PrP expression in cultured neural cells. This is the first time a mutant protein genetically linked to a neurodegenerative condition has been shown to cause cytopathic effects in vitro. Our studies will have important ramifications for other dominantly inherited neurodegenerative diseases whose genetic loci are now known, including Huntington's disease, some forms of Alzheimer's disease, and familial amyotrophic lateral sclerosis, as they may provide guidelines for creating in vitro models of these other disorders. We propose 1) to characterize the type of neuronal degeneration exhibited by these cells, 2) to analyze the cellular characteristics of the proliferating non-neuronal cells, 3) to adapt the paradigm we have developed for studying PrP in vitro to examine whether the phenotypes of cultured cells expressing mutant and wild-type APP differ, and 4) to identify the essential pathogenic domains of PrP and APP mutants in our experimental paradigm.